The electromagnetic rays to which this invention refers are often, but not necessarily, in the visible spectrum. The substantially point source emitters of this invention are suited to be disposed on a supporting object, of known size and shape, whose position and orientation are being determined in a three dimensional coordinate system from a determination of the locations of these emitters in the same coordinate system. They may also be disposed on one or more stationary and/or moving objects, of known size and shape, whose position(s) and orientation(s) are being tracked as it moves in space within a three dimensional coordinate system. In this use as a position and orientation determinant, the emitter is seen by a plurality of electromagnetic ray receptors, which are generally referred to herein as cameras.
The straight ray lines between a plurality of either the emitter(s), or the camera(s), or both, can be compared to straight reference lines to thus form a plurality of angles from which geometric information is obtained. This geometric information can, in turn, be used to determine the precise location(s) of the emitter(s) in space, and from these emitter locations, if the shape and size of the object are known, the position and orientation of the object on which the emitter(s) reside can be determined geometrically. Since the location of the emitter(s) can be tracked substantially continuously, or at least very frequently, movements of the object on which they are disposed can be tracked. The more frequently the locations of the emitters are determined, the more precise is the movement tracking ability of the system. Further, the smaller and less variant the emitter source of the electromagnetic rays, the more accurately can its location be determined. That is, the closer the emitter resembles a point source, which is in the same apparent location is space regardless of the angle from which it is viewed by the camera(s), the more accurate can its location be determined in a three dimensional coordinate system. It follows that the more accurate is the determination of the locations of the point sources, the more accurate is the determination of the position(s) and orientation(s) of the object(s) supporting the emitters.
Systems for tracking or determining the position and orientation of objects in space by means of measuring the angles intersected by beams of emitted electromagnetic radiation, either between two such beams, or between one such beam and a reference beam or line, are known. Reference is made to U.S. patent application Ser. No. 08/317,805, now U.S. Pat. No. 5,622,170, the entirety of which is incorporated herein by reference, which discloses a system for determining the spatial position and orientation of the object, by means of determining the location of emitters of electromagnetic rays which are disposed on its surface, using electro-optical sensors. In order to improve their accuracy, there is a desire for these emitters to more closely approximate or resemble a point source of emitted radiation.
As referred to herein, a point source emitter(s).congruent. are tiny, but finite dimensioned, radiating (sometimes luminous) bodies. These emitters, may themselves be the object that is being tracked. Alternatively, they may be of a size which is comparable to the size of the object on which they are disposed. However, in most applications, the emitters of this invention are usually of much smaller than the size of the object on which they are disposed. Commonly, these emitters are many orders of magnitude smaller than the volume of the three dimensional space in which the object is being located or tracked. Thus, in relation to the volume of space in which the tracked object may be moving, the emitter can be considered to have insignificant dimensions. Because of its small size, and without considering the particular emitter shapes described by the practice of this invention, the shape of the emitter can also be considered to be of no practical consequence. However, the emitters are preferably symmetrical and, ideally, should be spherical, or at least approaching spherical.
Various electro-optical methods have been described in the prior art to determine the location of a point-like emitter of electromagnetic energy within a three-dimensional (3-d) volume relative to some reference coordinate system. If there are multiple such emitters mounted at known locations on a substantially rigid object, determining the location coordinates for each emitter in the reference system can enable the determination of the position and orientation of the object, and therefore, if the size and shape of the object are known, the location of any particular point on the object can be determined in relation to the reference system.
One such method employs multiple angle-measuring optical sensors, wherein multiple sensors are used to determine the location of one or more emitters. The locations of a plurality of electromagnetic energy emitters may be determined with respect to one, two or three angular dimensions. With the plurality of electromagnetic energy sensors (cameras) appropriately situated in known or determinable locations within a coordinate, the 3-dimensional coordinates of the emitter(s) can be determined relative to that coordinate system with a significant degree of accuracy.
For example, each of two or more spaced-apart standard video cameras, situated at one or more known spatial positions, respectively, within a three dimensional space defined by some reference coordinate system, can observe the elevation and azimuth angles of the image of an infrared light emitting diode (LED) with respect to the local optical and mechanical axes, of each camera. An appropriately programmed electronic computer with appropriate software (both of which are per se conventional as regards the instant invention) can convert those angles and the position coordinates of the cameras into 3-dimensional rectangular coordinates of the location of each LED emitter with respect to each camera and therefore with respect to the reference coordinate system as a whole. Alternatively, a plurality of two or three or more, optical angular position sensors, for example, situated appropriately with respect to each other and at known, or determinable, positions in the coordinate system, can measure the location of each LED emitter. This operation of determining the three dimensional coordinates of a point radiating source is referred to as digitizing that point in space.
A number of such electromagnetic sensors have been described in published literature and have been used in spaced apart pairs or triples to determine the location of an electromagnetic radiation emitter in 3-dimensional space. A commercially available example is the FlashPoint 5000 system, manufactured by Image Guided Technologies of Boulder, Colo. That and other examples of systems using linear (one-dimensional) detectors are described in the following references to the state of the prior art:
FlashPoint 5000 Users Manual; Image Guided Technologies, Inc., Boulder, Colo., 1996.
H. Fuchs, J. Duran, B. Johnson, and Zvi. M. Kedem; "Acquisition and Modeling of Human Body Form Data", Proc. SPIE, v. 166, 1978, p 94-102.
Jean-Claude Reymond, Jean-Luc Hidalgo; A System for monitoring the movements of one or more point sources of luminous radiation.congruent., U.S. Pat. No. 4,209,254, Jun. 24, 1980.
Y. Yamashita, N. Suzuki, M. Oshima; "Three-Dimensional Stereometric Measurement System Using Optical Scanners, Cylindrical Lenses, and Line Sensors", Proc. SPIE, v. 361, 1983, p. 67-73.
F. Mesqui, F. Kaeser, and P. Fischer; "real-time, non-invasive recording and 3-d display of the functional movements of an arbitrary mandible point", SPIE Biostereometrics 602 (1985) p 77-84.
Sharon S. Welch, Kevin J. Shelton, and James I. Clemmons; A Optical position measurement for a large gap magnetic suspension system.congruent., Proc. of the 37th International Instrumentation Symposium, San Diego, May 5-9 , 1991, p. 163-182.
Waldean A. Schulz; A Method and apparatus for three-dimensional non-contact shape sensing.congruent., U.S. Pat. No. 5,198,877, issued Mar. 30, 1993.
Farhad Daghighian; A Optical position sensing with duolateral photoeffect diodes.congruent., Sensors, 1994 November, p. 31-39. Examples of systems using two-dimensional detectors are found in the following references, which reflect the state of the prior art:
U.S. Pat. No. 4,896,673 by Rose et al. PA1 U.S. Pat. No. 4,836,788 by Baumrind et al.
Provided there is a way to distinguish between the emissions of multiple electromagnetic energy emitters (for example LED's) that are mounted on a substantially rigid object, of known size and shape, the position and orientation of the object can be geometrically derived from the determined locations of the several emitters. To at least a limited extent, even the shape of an unknown object can be determined if a sufficient number of LED emitters are attached to each of its surface (see for example the '788 patent cited above). The more complicated the shape of the object, the more emitters are required to define its shape. Each emitter may be distinguished by a unique emission wavelength, by its relative location in some unambiguous geometrical pattern of emitters, or by its ordinal position in a serial emitting sequence. It is possible that other means of distinguishing between several emitters are known or will be discovered in time.
To date, in the field of tracking objects moving in space, the electromagnetic energy emitters, whose locations are to be determined, have usually been visible or infrared, light emitting diodes (LED's). Other wavelengths of electromagnetic radiation, in addition to the visible spectrum, are also well suited to use in this environment. The specific wavelength of the emitted radiation is not a limitation on the practice or scope of this invention.
Unfortunately, a conventional LED, which radiates in any given, predetermined wavelength, has several drawbacks to its use as a point source electromagnetic energy emitter. At the present time, electromagnetic energy emitting, semiconductor chips (LED's) are conventionally disposed in a protective, substantially transparent epoxy envelope. Protecting the semiconductor chip, and its electrical connections, are extremely important. However, One drawback of this necessary epoxy envelope is that it refracts the light rays that are being emitted by the chip, which shifts the apparent optical location of the chip. Viewing the chip through the epoxy envelope from different angles generally shifts its apparent location because of this diffraction.
A second drawback is that the LED semiconductor chip, which may be as large as 1 millimeter square and is usually mounted on a partially reflecting surface, does not radiate uniformly, even without the protective epoxy coating. This can cause an apparent shift in the location of the centroid of illumination as the cameras' view of the chip is rotated about the geometrical centroid of the chip. Furthermore, the chip typically has attached to its top, an electrical contact wire or metallic strip, which partially eclipses (or reflects) the light from part of the chip and introduces asymmetry into the radiation pattern. These effects limit the accuracy and repeatability of precisely locating the chip optically, particularly if the goal is to determine a coordinate that has a smaller margin of error than the size of the chip.
A third drawback occurs when the LED's are sequentially flashed as a way to unambiguously identify individual LED's. The flashing may generate electromagnetic interference unless the waveform of the current flow through each LED is carefully controlled Even in that case, the wires to the LED's tend to act as antennae, transmitting out other electronic noise from the control box, which in practice generates high frequency control signals which are often picked up by the electromagnetic sensor assembly.
A fourth drawback exists when the LED's are used within a surgical (medical) environment. The electrical current driving each LED must be very well isolated from ground, from the patient, and from all other electrical currents, including the electrical currents driving other LED's and/or other functions. Failure to completely isolate these electric currents can cause serious difficulties and even injury.
A fifth drawback exists when the LED's are used within a medical nuclear magnetic resonance imager (MRI). The metal electrical leads, the currents flowing through them, and the metallic case or heat sink (if used) deforms the MRI=s magnetic fields and thereby can warp the image of the patient.
A sixth problem occurs in designing a practical probe, a surgical instrument, or other such object, with LED's mounted on it. To reduce maintenance costs, the design should allow a burned-out LED to be replaced (and preferably to be replaced easily). Otherwise every time an LED becomes dysfunctional, the whole object/probe must be discarded and replaced. Generally, making the LED replaceable means using some kind of socket to house the LED's, which takes up additional space on the object and increases the difficulty of maintaining accurate placement of the energy emitting chip. That is, there must be a means to insure that a replacement LED chip is positioned at exactly the same effective location as the original and that all other relevant characteristics remain unchanged. If this is not accomplished, the system must be recalibrated every time an LED is replaced.
An LED as a generator of light is not a problem. In fact, an LED can be flashed much faster than an incandescent source. An LED is nearly monochromatic, allowing the position measurement sensors to use a narrow-band filter to cut out most interfering background illumination. LED's are inexpensive and have a long life. A laser diode, which is a very special kind of LED, is especially good for coupling light into an optical fiber. These characteristics, and the present lack of a suitable alternative light source, suggest that LED's, or the special case laser diodes, can be expected to continue to play a role in the determination of the position and orientation of objects in space. Therefore, solutions to the above problems must be determined.
Optical fibers, or bundles of optical fibers, are a good way to transport light from a source to a destination. However, they possess a major drawback to their use as light source, in the types of systems that are used for determining the locations of electromagnetic emitters, such as those described above. The drawback is that when the optical fiber end is considered to be the light source, the cone of light emitted from the end of a fiber (or fiber bundle) has an inconveniently narrow apex, or conical, angle. For the purposes of tracking an object which may have an arbitrary orientation, the light emitters on the object should ideally radiate uniformly over at least a substantial portion of a fall hemisphere, preferably at least the entire full hemisphere, and ideally, approaching a substantial whole of a sphere.